Process control systems, like those used in chemical, petroleum or other processes, typically include one or more process controllers and input/output (I/O) devices communicatively coupled to at least one host or operator workstation and to one or more field devices via analog, digital or combined analog/digital buses. The field devices, may be, for example, valves, valve positioners, switches and transmitters (e.g., temperature, pressure and flow rate sensors). These field devices may perform process control functions within the process such as opening or closing valves and measuring process control parameters. The process controllers receive signals indicative of process measurements made by the field devices, process this information to implement a control routine, and generate control signals that are sent over the buses or other communication lines to the field devices to control the operation of the process. In this manner, the process controllers may execute and coordinate control strategies using the field devices via the buses and/or other communication links.
Process information from the field devices and the controllers may be made available to one or more applications (i.e., software routines, programs, etc.) executed by the operator workstation (e.g., a processor-based system) to enable an operator to perform desired functions with respect to the process, such as viewing the current state of the process (e.g., via a graphical user interface), evaluating the process, modifying the operation of the process (e.g., via a visual object diagram), etc. Many process control systems also include one or more application stations (e.g., workstations). Typically, these application stations are implemented using a personal computer, laptop, or the like that is communicatively coupled to the controllers, operator workstations, and other systems within the process control system via a local area network (LAN). Each application station may include a graphical user interface that displays the process control information including values of process variables, values of quality parameters associated with the process, process fault detection information, and/or process status information.
Process control systems involving batch processes typically process a common set of raw materials or feedstock through various numbers of stages or steps as a batch to produce a product. One or more steps or stages of a batch process may be performed in the same equipment, such as a processing tank, reactor, or other type of processing equipment. The feedstock is fed into the reactor at various stages of the batch process from various other tanks such as storage tanks and other reactors. Process information from field devices and controllers coupled to the storage tanks and reactors may be made available to one or more applications executed by the operator workstation to enable an operator to perforin desired functions with respect to the batch process.
To control the quality of a batch process product, it is important to understand exactly what is happening at each stage of the process. Understanding the properties of the feedstock that is being fed from the storage tanks into the various processing tanks at different stages of the process is one factor to consider in determining the quality of the final product. For example, the feedstock may have varying properties depending on a variety of factors including source, time of year, age, storage conditions, etc. Without a clear understanding of feedstock properties, it may be difficult to control the quality of the final batch process product. For example, any changes in the properties of the feedstock, even within accepted limits, can impact the reactor operation and quality parameters in the final batch process product.
In some processes, feedstock is blended as it is delivered into the storage tank to achieve uniform properties for the feedstock as it is pumped out of the tank and into a reactor. Under turbulent conditions, blending occurs via convection and turbulent dispersion. Dispersion may be created by different equipment: stirred tanks, jet mixers and ultrasound mixers. In processes where the feedstock is blended, continuous calculation of the storage tank pump out concentration may be achieved by also measuring the storage tank's input concentration, input flow, pump out flow and storage tank level (or weight). Processes using blended feedstock also assume perfect blending.
Batch process operations typically do not use blended feedstock. Storage tanks in batch processes are usually loaded periodically from trucks or from reactors and do not employ mixers to mix the feedstock. Deliveries of additional feedstock may be accompanied by analysis data that allows the batch process to at least account for the input feedstock that is added to the previous feedstock. However, as a storage tank is depleted, more feedstock is added. This input feedstock likely includes slightly different properties than the old feedstock. In storage tanks without mixers, a certain amount of stratification or “layering” of the feedstock may occur as new input feedstock is added to the old feedstock. In addition to layering, a certain amount of mixing between the layers may also occur due to naturally-occurring turbulence and other factors, but complete blending is not possible in storage tanks without mixers. As such, it is difficult to predict the exact properties of the pump out feedstock that is fed into a batch process reactor after it leaves the storage tank.